Uranium cloud point extraction and its measurement in water samples, especially sea water

Dissertation for Master's degree

in advanced chemical engineering

Abstract

A new method based on point cloud extraction to extract uranium from water samples and measure it using alpha spectroscopy based on liquid scintillation has been investigated. Uranium extraction from water samples in the presence of a number of available surfactants and chelating reagents has been done using this method. The chelating ions were extracted into the surfactant-rich phase by forming a complex at high temperatures above the cloud point. The complete separation of the phases was done using a centrifuge. After separating the phases, the amount of uranium in the aqueous phase was measured using a liquid scintillation counter, and in this way the extraction efficiency was calculated and the suitable ligand and surfactant were selected. Also, the effective factors on the spot extraction process of uranium cloud were identified and the effect of these factors (environmental pH, ligand concentration, surfactant concentration, interference of interfering ions, temperature and equilibrium time, retention time in the centrifuge) was evaluated. Optimum conditions for extracting uranium from aqueous solutions were determined using cloud point extraction method. In the examined conditions, the extraction efficiency is high (99%) and the calibration curve is linear in the range of 6.25 x 10-4 to 0.1 ?g/liter. Also, the correlation coefficient of 0.99 indicates a suitable linear relationship between the concentration of species and the analytical signal. The calculated limit of detection (LOD), which is 3 times the ratio of the standard deviation of the witness to the slope of the calibration curve (m/Sd3), is equal to 0.1 ?g/liter per 50 milliliters of sample. The preconcentration factor, which is calculated from the ratio of the desired species concentration in the surfactant phase to its concentration in the aqueous phase, was 48.5.

The proposed method for measuring uranium In real water samples, two samples of tap water and sea water were measured, and the recovery was between 98 and 113 percent. The results showed the accuracy and practicality of this type of analysis. Industry, agriculture and the environment play an important role. Most people in the world suffer from water pollution [1]. The contamination of water systems with organic substances, heavy metals and radioactive substances is one of the most important problems of industrial or semi-industrial societies. In recent years, many efforts have been made to introduce and develop methods to remove pollution from water sources. But water pollution is a problem that is expanding and increasing with the progress of industrial activities as well as agriculture. Many pollutants even in very low concentrations have harmful effects on human health and the environment.

In recent decades, the development of methods for extracting or separating organic pollutants that are present in a very small amount in environmental sources has received special attention. The many issues and problems that the presence of metal ions in the samples brings, increases the necessity of detecting and measuring them in small amounts. Since in many cases the amount of these metal ions is less than the detection limit of analytical devices and due to the complexity of the sample tissue and the presence of disturbing species, especially in biological and environmental samples, direct measurement of metal ions is not possible. Therefore, it is necessary to perform sample preparation operations, including extraction and pre-concentration operations, in order to change the texture of the sample and increase the concentration of the desired species in a way that can be detected and measured with analytical devices.

In recent years, many efforts have been made to invent new methods to measure small amounts of species in various samples, and significant progress has been made in this field. Sample preparation, which includes converting the real sample tissue into a tissue that is suitable for analysis by separation technique or other methods, is generally considered the most time-consuming step of analysis. Sample preparation can be the main source of low precision and accuracy of the method.In general, the preparation of the sample is done in order to achieve the following goals:

1- Removing potential interferences from the sample with the aim of increasing the selectivity of the method,

2- Preconcentrating the desired species, increasing the sensitivity of the method and, if necessary, transforming the species into a form that is more suitable for isolation or identification.

3- Preparing a repeatable and strong method which is independent of the changes in the sample texture.

The most basic stage of sample preparation is the extraction process, which is used to separate and preconcentrate small amounts of species from the sample texture (solid, liquid or gas). An ideal extraction method is a method that is fast, simple, repeatable and cheap, and enables the quantitative recovery of the desired species without losing or destroying them. It should be done with a small sample volume, it should have high selectivity, the use of solvent should be minimized, and it should be able to be automated and used continuously with analysis systems, and finally, it should not need to concentrate and reduce the volume of the extractive phase.

In the first chapter of this thesis, general information about uranium and its applications and the necessity of using cloud point extraction method for its preconcentration has been discussed.

In the second chapter, the theory of cloud point extraction is given in detail, and then there is an overview of the research done in this field.

In the third chapter, the research method and the way of conducting the experiments are explained. In this chapter, the effect of various factors (environmental pH, ligand concentration, surfactant concentration, interference of disturbing ions, temperature and equilibrium time, retention time in the centrifuge) on the efficiency of uranium extraction has been discussed and investigated.

In the fourth chapter, the results of cloud point extraction experiments and the effect of various parameters on it have been stated.

The fifth chapter summarizes and presents suggestions regarding It is devoted to research and its future. 1-2 Uranium and its compounds in the nuclear fuel cycle 1-2-1 Introduction Uranium with the chemical symbol U and atomic number 92 is a radioactive metal element [1] that is part of the actinide group and is one of the heaviest elements in nature. This element belongs to natural radioactive materials that exist in very small amounts in rocks, soil, water, plants, and human and animal bodies. Pure uranium is a heavy metal with high density and light silver color, which can have different chemical forms, but it is found in nature in the form of uranium oxides, the most stable of which is triuranium octaoxide (8O3U). Uranium has little radioactive activity and does not play a significant role in natural background radiation.

Abstract

A simple method using cloud-point extraction followed by alpha liquid scintillation spectrometry was developed for simultaneous separation and determination of trace amounts of uranium (VI) in water samples. The extraction of analyte from water samples was performed in the presence of different chelating agents and surfactants. The extraction methodology used is based on the formation of soluble metal complexes in a micellar phase of surfactant. The metal ion complex are then extracted into the surfactant-rich phase at a temperature above the cloud-point temperature. After phase separation, the upper aqueous phase was removed and the U(VI) concentration was determined by liquid scintillation spectrometry. The different variables affecting the complexation and extraction conditions, cloud point pre-concentration and subsequent performance of LSC were optimized. Under the optimum conditions 8HQ as chelating reagent, Triton X-114 as surfactant, pH=6, surfactant concentration of 0.25%(w/v), chelating agent/uranium 8Hq/U(VI) molar ratio of 30, equilibrium temperature of 50?C, incubation time of 30 min, centrifugation time 15 min at 5000 rpm, uranium concentration 30 ppm) the calibration graphs were linear in the range of 6.25×10-4 to 0.1 ?g L-1 of uranium(VI) ion and the detection limit (DL) of the method is 0.1 ?g L-1.